Conventional etching methods typically include the steps of applying a photoresist to an article having an etchable film, for example, aluminum or amorphous silicon, supported on a substrate. The photoresist is selectively exposed and developed to form a mask layer over the film. The portions of the film not covered by the mask layer are etched to remove the exposed portions of the etchable film. Finally, the mask layer is removed, for example, by use of a solvent.
Known etching processes include "wet" etching. In a wet etching process, the article to be etched is exposed to an etching solution, such as a solution including hydrofluoric acid (HF). Wet etching processes are described, for example, in S. Ghandhi, VLSI Fabrication Principles (2d. Ed., John Wiley & Sons, Inc., New York, N.Y. 1994), pp. 589-613.
Wet etching processes, however, suffer from various disadvantages. For example, it is usually impracticable to change the etching solution after each article is processed; etching solutions typically are changed once per week. As a result, particles and residues accumulate in the etching solution. These accumulations tend to reduce the quality of the etching over time. Another problem is lack of uniformity. Wet etching processes are often incapable of uniformly removing an exposed etchable film over the entire exposed surface of the article, and in particular can suffer from undercutting of the etchable film. Such processes furthermore require a substantial area for the required etching tanks, rinsing tanks, drying apparatus, etc. Finally, wet etching processes, particularly those using strong etchant solutions and/or organic solvents, are environmentally hostile and generate significant disposal problems.
Dry etching methods avoid many of the problems associated with wet etching methods in the production of integrated circuits. Dry etching methods are described, for example, in the above-noted publication at pp. 613-624. Such methods include dry physical etching methods, such as ion beam etching and sputter etching, and dry chemical etching. Dry etching methods typically utilize fewer chemicals and in smaller quantities, are readily automated, and give rise to fewer disposal problems.
In producing integrated circuits, it is important that the etching process employed produce a substantially vertical profile in the etched film. FIGS. 1 and 2 illustrate a conventional dry etching process. In FIG. 1, an article including a substrate 10, an etchable film 12 (for example, an aluminum film or an amorphous silicon film), and a mask layer 14 in which a pattern 16 is formed is exposed to an etchant gas. Alternatively, the article can be subjected to plasma etching. Mask layer 14 has a mask surface 18 which forms a first etch angle .theta..sub.1, with respect to etchable film 12. After removal of the exposed portion of etchable film 12 by the selected etching process, the surface 20 of the etchable film 12 forms a second etch angle .theta..sub.2 with respect to substrate 10, as shown in FIG. 2. Surface 20 preferably forms an angle .theta..sub.2 of about 85.degree.-90.degree., preferably approximately 90.degree..
In the illustrated process, etchable film 12 etches at rate R.sub.F, and mask layer 14 etches at rate R.sub.M. The ratio of the two rates, R.sub.F /R.sub.M, preferably is substantially greater than one, and is typically about 4 to 10. The high etch rate of film 12 with respect to mask layer 14 enables formation of a substantially vertical etching profile in film 12, as shown in FIG. 2.
In other applications, however, such as the production of flat panel displays (FPDs), it is important to produce a profile which is inclined with respect to the vertical, i.e., a tapered profile. As shown in FIG. 3, it may be desired to produce an etch angle .theta..sub.2 in etchable layer 12 which is between about 15.degree. and 60.degree.. To produce such a tapered profile, it is desirable that the respective etch rates of film 12 and mask layer 14 are more nearly equal in value, preferably such that the ratio R.sub.F /R.sub.M is less than about 2, very preferably between about 1 and 2.
Problems arise in producing tapered profiles. It is difficult, for example, to achieve the desired low ratio of R.sub.F to R.sub.M with known dry etching techniques. More significantly, it is difficult to completely etch the interface 22 at which etchable film 12 and mask layer 14 meet. The difficulty arises due the presence of oxidation products, such as alumina (Al.sub.2 O.sub.3) in the case of an aluminum film 12 or silicon dioxide (SiO.sub.2) in the case of an amorphous silicon film 12, which may be present in a surface region 24 of film 12, and/or the presence of mask residues. If interface 22 is etched at a lower rate than mask 14 and film 12, a "ledge" 26 is formed at interface 22, as shown in FIG. 4, and a continuous tapered contour cannot be produced.
In known processes for producing a tapered etch profile, chlorine-based etching chemistry is frequently employed. For example, when etchable film 12 is an aluminum film, boron chloride (BCl.sub.3) and chlorine gas (Cl.sub.2) are employed. Initially, BCl.sub.3 removes the Al.sub.2 O.sub.3 oxidation products from surface region 24 of the exposed aluminum film 12. The mixture of BCl.sub.3 and Cl.sub.2 is then used to etch aluminum film 12 and mask 14. However, the gas mixture does not remove the Al.sub.2 O.sub.3 from interface 22 as effectively as it removes film 12 and mask 14. This again results in formation of a ledge 26, preventing a smooth etch profile from being formed across film 12 and mask layer 14.
Processes for etching a film 12 of other known materials also encounter problems with ledge formation. When film 12 is an amorphous silicon film, BCl.sub.3 typically removes the silicon oxidation products from surface region 24 of exposed silicon film 12. Cl.sub.2 is then used to etch silicon film 12. Again, Cl.sub.2 does not remove the silicon from interface 22 at the same rate as it removes film 12 and mask layer 14, and a tapered contour cannot readily be formed.
A need exists for a dry etching method that enables production of an etched article, such as a FPD, having a smooth, continuous tapered etch profile which is substantially free of ledges and other irregularities.